The present invention pertains generally to photoconducting devices and more particularly to photoconducting devices having sub-nanosecond response times and photoconductive gains greater than unity.
In recent years interest has developed in bulk semiconductor photoconductors with sub-nanosecond carrier recombination lifetimes for a number of instrumentation applications, including use as photodetectors. Other applications for picosecond photoconductors include generation of fast rising or short duration electrical pulses and picosecond electronic gating. For many of these applications picosecond photoconductors have a significant advantage over conventional high-speed semiconductor devices. Because the speed of response is a property of the material (carrier recombination lifetime) and not of the carrier transit-time, speed is independent of both bias voltage and the dimensions of the device. This permits use in a variety of circuit functions not accessible to conventional devices. In general, material requirements for these applications are high resistivity (low dark current), high carrier mobility (high responsivity), and carrier recombination lifetimes in the picosecond domain determined reproducibly by device fabrication processes. A number of materials have previously been investigated for application as picosecond photoconductors. Amorphous Si and radiation-damaged Si have both shown fast response (4 and 8 picoseconds, respectively) and high resistivity but poor sensitivity due to low carrier mobility (1 and 10 cm.sup.2 /V-sec, respectively). Semi-insulating GaAs has shown fast response (50 picoseconds) and high resistivity but poorer sensitivity than expected from bulk carrier mobility. Proton-bombarded, semi-insulating InP has shown fast response (less than 100 picoseconds) with relatively high carrier mobility (600 cm.sup.2 /V-sec) but large dark current.
Semi-insulating InP without additional damage has shown response less than 100 picoseconds, with carrier mobility of 2000 cm.sup.2 /V-sec, and low dark current. The present invention focusses on this material.
One additional advantage of bulk photoconductors compared to photodiodes for use as photodetectors is their potential for achieving photoconductive gain greater than one. Photoconductive gain is defined as the number of electrons delivered to the external circuit per incident photon on the detector. This means that photoconductive gain is directly proportional to current responsivity (amps/watt). Until this present invention, gain has not been reported in a picosecond photoconductor. Photoconductive gains greater than unity have been reported in photoconductive detectors in which the speed of response is controlled by the transit time of holes across the active region, as opposed to recombination within the active region. These devices can yield maximum gains equal to the ratio of electron to hole drift velocities (.about.2 for Si, .about.10 for GaAs and InP). However, because the mechanism controlling speed of response is carrier transit-time and not bulk recombination, these devices cannot perform the range of circuit functions accessible to bulk photoconductors as described above. Also, bulk photoconductors can in principle achieve higher gains because carrier recombination and transit-time are independent so that speed, gain, and device dimensions can be traded off against each other depending on the desired application. (For example: a contact spacing of one micron in an InP photoconductor with peak electron velocity of 2.6.times.10.sup.7 cm/sec and a carrier recombination lifetime of 100 picoseconds implies a gain of 25 with a speed of response of 100 picoseconds).